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When you manually defined the task duration as 1 week, MS project behind the scenes changes the task to Fixed Duration. Meaning, that pretty much the only way the task duration will change will be if you manually alter it.
The solution is pretty simple. Change the task type to Fixed Units. Once you change the task type, change the Work value from lets say 40, to 39, then back to 40. This change will force Project to recalculate the duration as explained in the additional info below.
Project has an internal formula “Duration = Work / Availability” where availability is resource assignment units equated to hours of work time available per day. This formula must always be in balance. When you define a task as Fixed Work, you are locking the Work portion of the formula, meaning if you manually change availability, duration is recalculated. It works the same for Fixed Units (availability), meaning the Availability is locked and if you change the amount of work, the Duration will be recalculated.
Generally speaking, if you are working on projects where duration should be driven by work and resource availability, you will probably want to set your task types to Fixed Units. For example, Bob is available to the project at 60% so by setting the task type to Fixed Units, you’re telling Project not to change Bob’s task assignment units value.
Hope that helps! 🙂